1. Field of the Invention
The present invention relates to a device for activating an air bag, for occupant protection, in the event of a vehicle collision.
2. Prior Art
FIG. 1 shows the configuration of a prior art air bag activation device. In the figure, reference numeral 100 is a power supply for igniting an air bag, which usually comprises a battery and a backup capacitor. Reference numeral 101 is an acceleration sensor, generally known as a mechanical safing sensor, for mechanically detecting impact severity. Reference numeral 102 is an acceleration sensor, generally known as a G sensor, for electronically detecting impact severity. Reference numerals 103 and 104 are acceleration sensors mounted at the right and left front ends of a vehicle and used to electronically detect the severity of the impact applied to the vehicle; this type of sensor is generally known as a front sensor.
Further, reference numeral 105 is an igniter (hereinafter called the squib) which, when energized, ignites and causes a chemical to undergo a chemical reaction to generate gas. The gas is used to inflate the air bag. Reference numeral 106 is a microcomputer which processes the acceleration signals supplied from the G sensor 102 and the right and left front sensors 103 and 104, determines whether the impact applied to the vehicle is of a severity that warrants the activation of the air bag, and if it is warranted, produces a signal for igniting the squib.
The section enclosed by dashed lines in FIG. 1 is a squib ignition circuit which is usually implemented using an integrated ASIC. The ignition circuit 107 contains a decoder 108 for decoding the signal supplied from the microcomputer 106, and for supplying a conduction signal to the bases of transistors 109 and 110 which together constitute a squib driver. The ignition circuit 107 further contains a receiving circuit 111 for receiving signals from the right and left front sensors 103 and 104. Outputs of the receiving circuit 111 are sent to the microcomputer 106 which detects the mode of crash based on such factors as the difference between the impact severities detected by the right and left sensors. The detected information is used to determine the mode of air bag deployment.
In the air bag activation circuit described above, the acceleration (G) signals from the G sensor 102 and the right and left front sensors 103 and 104 are processed by the microcomputer 106 to determine whether the impact is due to a crash and to determine the mode of crash. If it is determined that a crash has occurred, the microcomputer 106 produces a signal to turn on the transistors 109 and 110; this signal is decoded by the decoder 108 and applied to the bases of the transistors 109 and 110. As a result, the transistors 109 and 110 are turned on, and an ignition current is thus supplied to the squib.
On the other hand, the mechanical safing sensor 101 mechanically detects impact severity, independently of the G sensor 102 and the right and left front sensors 103 and 104, and if the impact severity is greater than a predetermined value, the contact is closed to form a state in which a current can be supplied to the transistors 109 and 110. The magnitude of impact severity that causes the mechanical safing sensor 101 to close the contact is chosen to be smaller than the magnitude of impact with which the G sensor 102 determines that a crash has occurred.
The reason is that the mechanical sensor 101 is provided primarily for safing purposes, that is, to prevent an erroneous activation of the air bag due to malfunctioning of the G sensor 102 or the microcomputer 106; more specifically, when an impact is distinctly detected by the mechanical sensor 101 and, on top of that, the G sensor 102 detects the occurrence of a crash, then the current is supplied to the transistors 109 and 110.
Accordingly, in this ignition circuit, if the G sensor 102 or the microcomputer 106 malfunctions due to noise caused by an external electric wave or the like, and outputs a signal indicating the occurrence of a crash when actually a crash has not occurred, since such noise does not cause the mechanical safing sensor 101 to close its contact, an erroneous activation of the air bag is prevented.
As described above, in the prior art air bag activation circuit, a safing system is constructed by providing the mechanical sensor in addition to the electronic sensors in order to prevent an erroneous activation of the air bag due to malfunctioning of the electronic G sensor, the microcomputer, or the like.